1. Field of the Invention
This invention relates to coatings for high temperature solar thermal collectors. More particularly, this invention relates to coatings for high temperature, especially central receiver type, solar thermal collectors.
2. The Prior Art
Utilization of solar energy by heat absorption is well known. Typically, a tube or plate is used to absorb the solar radiation and to transfer the energy absorbed to a fluid with which it is in heat transfer relationship.
In low temperature solar systems (those reaching temperatures of approximately 220.degree. F.), generally a flat plate, painted black, is used to directly collect the incident solar radiation. The black surface is an inexpensive technique for increasing the efficiency of such systems by increasing the relatively short wave radiation, i.e. below about 1.5 microns, which is absorbed by the tube or plate. An example of such a low temperature flat plate collector can be found in U.S. Pat. No. 3,952,725.
In intermediate temperature systems (those reaching temperature of generally up to about 550.degree. F.), reflective surfaces or mirrors or Fresnel lenses and the like are employed to concentrate the incident solar radiation for absorption by the solar thermal collector. The solar radiation incident on the mirror, for example, is focused onto a tube or plate for absorption thereby and subsequent transfer of the energy absorbed to a fluid with which the tube or plate is in contact. Typically, in intermediate temperature systems the extent of solar concentration is in the range of about 10 to about 40 times the normal incident solar radiation.
In high temperature solar systems (those reaching temperatures in excess of about 550.degree. F., and generally above 900.degree. F.), heliotropic mechanisms are employed in conjunction with reflecting surfaces or mirrors so as to concentrate the normal solar radiation incident over a given area so that the radiation impinging on the tube or plate of the solar collector is from about 100 to about 1500 times greater than the normal incident solar radiation. In such a system, for example, a central receiver of concentrated solar radiation can be located in a tower centrally placed within a field of solar tracking reflectors.
For efficient operation of the intermediate and high temperature solar collectors, it has heretofore been considered most important that the coating on the collector have a solar absorptance, .alpha., as high as possible, the limit thereof being that of a theoretical black body or 100% with .alpha.=1.0, and an emittance, .epsilon., in the infrared wavelength range (i.e., greater than 3 microns) as low as possible, preferably below 0.2.
Numerous attempts have been made in the past to develop "selective absorber coatings" for solar collectors. By "selective absorber coatings" is meant coatings which display a high absorptance, .alpha., over most of the solar spectrum and a high reflectance in the near infrared to minimize re-radiation losses. Selective absorbers are said to have high .alpha./.epsilon. ratios. One such coating is disclosed, for example, in U.S. Pat. No. 3,958,554.
There are numerous drawbacks associated with selective coatings. The primary deficiency of presently developed selective coatings is that although they are very effective absorbers through most of the solar spectrum, they effectively cut off the tail end of the higher wavelength region of the solar spectrum with the result that although re-radiation losses in the infrared are kept at a minimum due to the spectral selectivity of the coating, reflection losses at the tail end of the solar spectrum are greater than the energy gain realized from reduction in radiation losses. For example, a typical multilayer, state-of-the-art, selective collector coating will have a solar absorptance above 0.9 from 0.4 to 1.5 microns, but between 1.5 to 3.0 microns the solar absorptance typically decreases from 0.9 to 0.2 or below. Indeed, the cut-off point from high to low absorptance generally occurs at approximately 1.5 microns, which is far too early in the solar spectrum. Consequently, some of the incident solar radiation is reflected by the coating. The net result is that the overall solar absorptance is only about 0.85 to 0.90 (or 85 to 90%). Hence, while the selective coating effectively cuts down on re-radiation losses due to the low emissivity in the infrared (.epsilon.=0.1), the selective coating lacks a high solar absorptance over the whole solar spectrum and about 10% to 15% of the solar radiation is lost due to reflection.
Selective coatings have other major drawbacks. Notably, multilayer compositions require close control of layer thickness. Many selective coatings lack "in-place" reparability and many possess the potential for interdiffusion of the coating materials with concomitant loss of selectivity.
In high temperature solar collector systems, the attendant problems of selective coatings are even more severe. Many coatings remain selective only so long as the composition and/or film thickness have not changed. Many selective coatings, however, are not chemically stable at the temperatures prevailing in high teperature collectors. Thin films are permeable to oxygen at high temperature conditions also, and oxidation of the substrate results in both increased emittance and coating failure due to peeling of the film.